Presently, B-dot sensors are widely used for high-power microwave (HPM) test and evaluation. They are, in general, composed of a metallic loop antenna or coil that interacts with the electromagnetic field; the metal in the antenna or coil results in unacceptably large field perturbations. As a consequence, the magnetic field measured by the B-dot sensor is not a true field, and it is often difficult or impossible to obtain reliable HPM test and evaluation (T&E) results with such B-dot sensors, particularly in confined spaces. In addition, B-dot sensors have a narrow bandwidth. To perform HPM T&E over a broad frequency bandwidth, several different B-dot sensors with complementary bandwidths are required. Thirdly, the signal produced by the B-dot depends on the time derivative of the HPM signal. Consequently low sensitivities are obtained at the crests and troughs of the waveform, while higher sensitivities are obtained in the regions in between. Finally, because the B-dot sensor measures time derivative, additional signal processing is necessary to obtain the amplitude and waveform of the external field.
The Hall probe is a convenient magnetic field sensor, used at room temperature. However, its sensitivity is several orders of magnitude poorer than that of the SQUID or atomic vapor cell. In addition, it has a narrow dynamic range and a very limited frequency bandwidth (DC-kHz).
Superconducting quantum-interference devices (SQUIDs) are the most sensitive magnetometers that are commercially available. The operating bandwidth of SQUIDs is typically from DC to a few GHz. However, SQUIDs must be operated at cryogenic temperatures, which are typically at or below −269° C. This requires that the SQUID be kept inside a cryogenic Dewar; thus an operational SQUID is very large, bulky, and has limited portability. The SQUID also contains metallic and superconducting components, which can interfere with the measurement of the electromagnetic field.
Atomic vapor cells are very sensitive magnetic field sensors, currently being developed by several research groups. A few of these groups have already demonstrated atomic vapor cells that have sensitivities exceeding those of SQUIDs. An atomic vapor cell requires an oven, which must keep the cell at a constant temperature, in order to produce atomic vapor. Although a state-of-the art atomic vapor cell uses a small oven, contained within the vapor cell device, vapor cells can only be used in limited applications, namely, those that do not alter the oven temperature.